Zeolites are widely used as sorbents in many applications that use the zeolite's ability to entrap liquids and gases. One potential application is the development of zeolite storage materials for gases. In addition, zeolites offer the possibility of selective separation of gases from mixed streams.
Zeolites are crystalline aluminosilicates with framework structures. The framework structure contains channels and cages of molecular dimensions. Cations and small molecules can reside on the face of the framework or within the pores, also called cages. Zeolite rho is a typical example, with a 3-dimensional network of alpha-cages (cubo-octahedra) which are connected to each other by octahedral prisms, or in other words, a body centered cubic structure of alpha cages. Selective blocking of pores in zeolites can be achieved by ion-exchanging the zeolites with different sized cations, thereby altering the sorption properties of the zeolite.
There have been several attempts to encapsulate hydrogen gas in various metal-exchanged zeolites. For example, Yoon and Heo (J. Phys. Chem., Vol. 96, pp. 4997-5000, 1992) studied encapsulation in Cs.sub.2.5 -zeolite A at pressures ranging from 10-129 atm (1.01-13.1 MPa) and temperatures ranging from 100-350.degree. C., and achieved a maximum amount of 871 .mu.mol/g of H.sub.2 encapsulation.
Weitkamp et al. (Proc. 9th Intl. Zeolite Conf., Montreal 1992, Eds. Ballmoos et al., Butterworth-Heinemann Pub., vol. 2, pp. 11-19) used various metal-exchanged zeolites including zeolite-rho. However, the largest amount of H.sub.2 that they were able to encapsulate was 410 .mu.mol/g using zeolite NaA at 300.degree. C. and 10.0 MPa after 15 min. They were also only able to encapsulate 22.3 .mu.mol/g of H.sub.2 using zeolite H-rho under the same conditions.
Efstathiou et al. (J. of Catalysts, vol. 135, pp. 135-146, 1992) studied H.sub.2 encapsulation in Cs, Ni, and Eu-exchanged zeolite A at 1 atm (0.1 MPa) and 37-300.degree. C. They achieved the largest amount of H.sub.2 encapsulation, 3.50 .mu.mol/g, using NaA at 300.degree. C.
Takaishi et al. (U.S. Pat. No. 4,466,812) disclose a hydrogen encapsulating zeolite material composed of a Na zeolite A ion-exchanged with cesium and a divalent metal. H.sub.2 encapsulation was performed at 300.degree. C. or less at pressures of 97 atm (9.8 MPa). No specific examples are provided for a composition containing Cd, although Cd is generally disclosed as one of the divalent metals.
What are needed are zeolite compositions capable of encapsulating hydrogen in larger amounts and at lower pressure than the prior art. In addition, a method of selectively removing hydrogen from hydrogen-containing process streams using said zeolite compositions is also needed. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the detailed description of the invention which hereinafter follows.